The nucleotide sequence of pSW200 ofErwinia stewartiiSW2 was determined. This plasmid is 4367 bp long, consisting of four mobilization genes,mobCABD,and an origin of replication homologous to those of ColE1-type plasmids. The plasmid also contains a region of forty-one 15-bp repeats. Deleting this region does not affect the stability or the copy number when maintained as sole plasmid in the cell. However, the plasmid is rapidly lost when a homoplasmid with the intact repeat region is introduced into the cell. The function of this region may provide pSW200 an advantage in competing with an incompatible plasmid in the cell.